Method for making toughened electrode-supported ceramic fuel cells

ABSTRACT

A solid oxide electrochemical device having a laminar composite electrode with improved electrochemical and mechanical performance, the laminar composite electrode comprising a porous support electrode layer, a thin and patterned structure layer, and a thin and dense electrolyte layer and methods for making.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 11/278,212filed with the U.S. Patent and Trademark Office on Mar. 31, 2006.

FIELD OF INVENTION

This invention relates to solid oxide electrochemical devices. Inparticular, this invention relates to solid oxide electrochemicaldevices having an electrode-support with improved structural properties.

BACKGROUND OF INVENTION

Solid oxide electrochemical devices have demonstrated great potentialfor future power generation with high efficiency and low emission. Suchsolid oxide electrochemical devices include solid oxide fuel cells(SOFCs) for power generation and solid oxide electrolyzers for chemical(e.g. H₂, O₂, and CO) production.

In an SOFC, stacks of fuel cells, each of which is capable of generatinga small amount of power, are connected together. Each cell is connectedto its neighboring cell with an interconnect, which serves as both acurrent collector and a channel for flowing gases to the electrodes. Twobasic cell constructions are used for SOFCs: electrolyte-supported cellsand electrode-supported cells. In electrode-supported cells, the supportelectrode functions include electrical flow path, mass transport path,and mechanical support. To satisfy these functions, thesupport-electrode must have sufficient conductive components, porosity,and strength.

Typical support-electrodes must be considerably thick to provide therequired mechanical strength and handling ability. Inelectrode-supported cell fabrication, the differences in sinteringdensification behavior and coefficient of thermal expansion (CTE) of theelectrode and electrolyte components result in non-flatness (such ascamber shape and edge ripples) of the cell. Generally, as supportelectrode thickness increases, the cell cambering tends to be reduced.The low mechanical strength and cell non-flatness in electrode-supportedcells can lead to cell fracture in fabrication, stack assembly, andoperation. While mechanical strength and cell flatness favor a thicksupport electrode, thick support-electrodes can restrict mass transportthrough the electrodes by limiting the oxygen transport in the supportcathode or fuel/product transport in the support anode. The limitationsin mass transport will lead to lower cell/stack performance, especiallyat high reactant utilizations for high efficiency. One approach toimprove mass transport through the thick electrode is to increaseporosity; however, the mechanical strength of the electrode will becompromised by too much porosity.

Previous attempts to address these problems have fallen short. Forexample, use of a composition gradient permits use of thicker and betterperforming anodes; however, the CTE mismatch between NiO and zirconiacreates challenges in fabricating large, flat electrode-supported cells.The large volume of Ni in the anode after reduction could also result increeping and sintering of the anode under the high operatingtemperatures of SOFCs.

An alternative structure is to form a continuous three-dimensionalnetwork with many microcomposite NiO and zirconia subelements inpatterns. The network improves the electrical connectivity and increasesthe strength of the overall structure; however, effectively controllingthe desired order of the subelements is difficult as they are vulnerableto distortion forces in the fabrication process.

Although these efforts show promise in improving the electrochemical andmechanical performance of electrode-supported cells, problems stillremain. Accordingly, there is a need for an electrode-supported cellwith improved electrochemical and mechanical performance.

SUMMARY OF INVENTION

This invention addresses the above-described need by providing anelectrode-supported electrochemical device comprising a laminarcomposite electrode and a second electrode. The laminar compositeelectrode comprises a porous support electrode, a thin and patternedstructure layer, and a thin and dense electrolyte, wherein the thin anddense electrolyte is adjacent to a first side of the porous supportelectrode and the thin and patterned structure layer is adjacent to asecond side of the porous support electrode, and a second electrodeadjacent the thin and dense electrolyte of the laminar compositeelectrode to form a complete cell.

In addition, this invention encompasses a method for making anelectrode-supported electrochemical device comprising fabricating alaminar composite electrode comprising a porous support electrode, athin and patterned structure layer, and a thin and dense electrolyte,co-firing the laminar composite electrode, and fabricating a secondelectrode on the surface of the dense electrolyte.

Other objects, features, and advantages of this invention will beapparent from the following detailed description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an electrode-supported electrochemicaldevice having a laminar composite electrode in accordance with anembodiment of this invention.

FIG. 2 is a partial plan view of patterned structural layers inaccordance with an embodiment of this invention.

FIG. 3 is an elevation view of the embodiment in FIG. 1 illustratingoperation of an electrode-supported electrochemical device.

DETAILED DESCRIPTION OF INVENTION

As summarized above, this invention encompasses an electrode-supportedelectrochemical device with improved electrochemical and mechanicalperformance. Embodiments of this invention are described in detail belowand illustrated in FIGS. 1-3.

A single solid oxide electrochemical device 10 having improvedelectrochemical and mechanical performance made in accordance with anembodiment of this invention is illustrated in FIG. 1. Moreparticularly, the solid oxide electrochemical device 10 in FIG. 1 is anSOFC, but it should be understood that this invention also encompassessolid oxide electrolyzers and solid oxide electrochemical pumps.Generally, the solid oxide electrochemical device 10 comprises anlaminar composite electrode 12, a second electrode 14, and first andsecond metallic interconnects 16 and 18. The metallic interconnects 16and 18 typically function as current collectors as well as a channel toform the desired flow fields.

The laminar composite electrode 12, illustrated in FIG. 1, comprises aporous support electrode 20, a thin patterned structural layer 22, and adense and thin electrolyte 24. The electrolyte 24 is on a first side ofthe porous support electrode 20 while the patterned structural layer 22is on a second side of the porous support electrode 20. The poroussupport electrode 20 preferably has a thickness in the range of about100 microns to about 3000 microns. The dense, thin electrolyte 24preferably has a thickness in the range of about 5 microns to about 40microns.

The porous support electrode 20 is comprised of either a cathode or ananode. In an embodiment, cathode materials are selected from the groupconsisting of lanthanum strontium manganites (LSM), praseodymiumstrontium manganites (PSM), lanthanum strontium ferrites (LSF),lanthanum strontium cobaltites (LSC), manganese-(Co, Cr, Ni) spinels,and a conductive pervoskite in the general form of ABO₃, where Acomprises at least one of the elements selected from the groupconsisting of La, Ce, Pr, Sr, Ca, and Ba; and B comprises at least oneof the elements selected from the group consisting of Mn, Fe, Co, Ni,Cu, and Zn. The cathode materials may comprise a single phase or acomposite with a ionic conductor such as doped zirconia or doped ceria.In an embodiment, the anode materials are selected from the groupconsisting of nickel cermet, copper/ceria cermet, and conductingceramics. Nickel cermets include nickel/yttria-stabilized zirconia(YSZ), nickel/ceria, and nickel/Scandia-stabilized zirconia. Conductingceramics include doped (La, Sr)TiO₃, doped ceria, doped LaNiO₃, dopedLaCrO₃, and doped niobates. Other cathode and anode materials may alsobe used and are known to those skilled in the art.

The dense, thin electrolyte 24 of the laminar composite electrode 12comprises a material selected from the group consisting of dopedzirconia, doped ceria, doped lanthanum gallate, and dopedBa(Sr)Ce(Zr)O₃, although other electrolyte materials may also be used.Such electrolyte materials are well known to those skilled in the art.

The thin, patterned structural layer 22 of the laminar compositeelectrode 12 functions as a frame to strengthen the solid oxideelectrochemical device 10 and keeps the solid oxide electrochemicaldevice 10 flat during sintering and operation. The thin, patternedstructural layer 22 has a pattern structured for providing a desired orpredetermined mass transport and electrical flow path through the poroussupport-electrode 20. Examples of patterns are illustrated in FIG. 2. Asshown in FIG. 2, open areas 26 without coverage by the patternedstructural layer 22 keep the mass and electrical flow paths toward theporous support-electrode 20. The land areas 28 with coverage by thepatterned structural layer 22 form an interweaved structure or an arrayof open areas defined by land areas. The thin, patterned structurallayer 22 desirably has material properties similar to the electrolyte24, creating a “symmetric” laminar composite electrode 12. The materialproperties that are desirably similar include composition, morphology,sintering behavior, and thermal expansion coefficients. Such materialsare selected from the group consisting of doped zirconia, doped ceria,doped lanthanum gallate, doped Ba(Sr)Ce(Zr)O₃, TiO₂, Al₂O₃, MnO_(x),MgO, and NiO. It is also desirable that the thin, patterned structurallayer 22 have a thickness similar to the electrolyte layer 24 (desirablyfrom about 1 micron to about 40 microns).

The second electrode 14 comprises either a cathode or an anode,depending on the composition of the laminar composite electrode 12. Whenthe laminar composite electrode 12 comprises a cathode, the secondelectrode 14 comprises an anode. Conversely, when the laminar compositeelectrode 12 comprises an anode, the second electrode 14 comprises acathode. In an embodiment, the second electrode 14 has a thickness inthe range of about 10 microns to about 100 microns. The cathodematerials are selected from the group consisting of lanthanum strontiummanganites (LSM), praseodymium strontium manganites (PSM), lanthanumstrontium ferrites (LSF), lanthanum strontium cobaltites (LSC),manganese-(Co, Cr, Ni) spinels, and a conductive pervoskite in thegeneral form of ABO₃, where A comprises at least one of the elementsselected from the group consisting of La, Ce, Pr, Sr, Ca, and Ba; and Bcomprises at least one of the elements selected from the groupconsisting of Mn, Fe, Co, Ni, Cu, and Zn. Cathode materials may comprisea single phase or a composite with ionic conductors such as dopedzirconia or doped ceria. The anode materials are selected from the groupconsisting of nickel cermet, copper/ceria cermet, and conductingceramics. Nickel cermets include nickel/YSZ, nickel/ceria, andnickel/scandia-stabilized zirconia. Conducting ceramics include doped(La, Sr)TiO₃, doped ceria, doped LaNiO₃, doped LaCrO₃, and dopedniobates. Other cathode and anode materials may also be used and areknown to those skilled in the art.

The metallic interconnects 16 and 18 are electrically connected to theelectrodes. The metallic interconnects 16 and 18 preferably are made ofelectrically conducting materials such as a metal plate or metal foil.In an embodiment, the metallic interconnects 16 and 18 are made ofmetals such as SS446 (stainless steel), SS430 (stainless steel), AL453(stainless steel), E-Brite (stainless steel) available from AlleghenyLudlum Corporation, Crofer 22 (Fe, Cr alloy) available from ThyssenKruppVDM, or Fecralloy (Fe, Cr, Al alloy) available from Goodfellow. Thelaminar composite electrode 12 and second electrode 14 are disposedbetween the metallic interconnects 16 and 18 to form a complete solidoxide electrochemical device module as illustrated in FIG. 1, althoughit is understood that the solid oxide electrochemical device 10 can takeother shapes. The metallic interconnects 16 and 18 connect the anode ofone device to the cathode of a second device creating a stack.

The solid oxide electrochemical device 10 may also comprise contactagents 30 and 32 between the electrodes 12 and 14 and metallicinterconnects 16 and 18 in a stack. The contacting agents 30 and 32 forman electrical path between the electrodes and the interconnects, thusforming electrical paths between cells in the stack. Contacting agents30 and 32 comprise conducting metal/ceramic materials in the form ofmesh, foam, felt, and paste.

FIG. 3 illustrates the solid oxide electrochemical device 10 of FIG. 1in operation. In operation, the solid oxide electrochemical device 10 isequipped with a gas inlet 34 for feeding gas along the gas flow pathbetween the laminar composite electrode 12 and metallic interconnect 16.The solid oxide electrochemical device 10 is also equipped with a secondgas inlet 36 for feeding a second gas along a flow path between thesecond electrode 14 and metallic interconnect 18.

This invention also encompasses a method for making anelectrode-supported solid oxide electrochemical device 10 with improvedelectrochemical and mechanical performance. A electrode-supportedbilayer comprising a support electrode 20 and thin electrolyte 24 on afirst side of the support electrode 20 is fabricated using processesknown to those of skill in the art. Such techniques include dryprocessing, tape casting, tape calendaring, and screen-printing. A thin,patterned structural layer 22 is fabricated on the second side of thesupport electrode 20 using techniques known to those of skill in theart, including techniques such as screen-printing, spraying, and slurrycoating. After being air-dried, the laminar composite electrode 12 isheated to burn out the organic and fired at a temperature appropriatefor the materials. The second electrode 14 is fabricated on the thinelectrolyte 24 using techniques known to those of skill in the art. Suchtechniques include screen-printing, spraying, and slurry coating. Theentire cell is fired at a temperature appropriate for the materials toform an electrode-supported solid oxide electrochemical device 10.

The present invention is further illustrated by the following examples,which is not to be construed in any way as imposing limitations upon thescope thereof. On the contrary, it is to be clearly understood thatresort may be had to various other embodiments, modifications, andequivalents thereof which, after reading the description therein, maysuggestion themselves to those skilled in the art without departing fromthe spirit of the present invention and/or the scope of the appendedclaims.

EXAMPLE 1

A laminar composite electrode comprises an anode-supported bilayer tape.The anode-supported bilayer tape comprises a thick 8YSZ-NiO supportanode and a thin YSZ electrolyte, made using processes known to those ofskill in the art. An example of such a process is tape-calendaring. Athin 8YSZ, zirconia with Al₂O₃ addition, or 3YSZ structural pattern isscreen-printed on the outer surface of the support anode at a thicknessranging from about 1 microns to about 7 microns and a width ranging fromabout 25 microns to about 125 microns. After being air-dried, the tapeis heated to burn out the organic material and fired at a temperature ofabout 1300° C. to about 1400° C. A LSM/YSZ cathode is screen printed onthe dense, thin YSZ electrolyte. The entire cell is fired at atemperature of about 1000° C. to about 1300° C. to form anelectrode-supported solid oxide electrochemical device.

EXAMPLE 2

A laminar composite electrode comprises an anode-supported bilayer tape.The anode-supported bilayer tape comprises a thick ceria-NiO supportanode and a thin ceria electrolyte, made using processes known to thoseof skill in the art. An example of such a process is tape-calendaring. Athin ceria structural pattern is screen-printed on the outer surface ofthe support anode at a thickness ranging from about 1 microns to about 7microns and a width ranging from about 25 microns to about 125 microns.After being air-dried, the tape is heated to burn out the organic andfired at a temperature of about 1300° C. to about 1400° C. A lanthanumferrite or lanthanum cobaltite cathode is screen printed on the dense,thin ceria electrolyte. The entire cell is fired at a temperature ofabout 900° C. to about 1200° C. to form an electrode-supported solidoxide electrochemical device.

EXAMPLE 3

A laminar composite electrode comprises an anode-supported bilayer tape.The anode-supported bilayer tape comprises a thick YSZ-NiO support anodeand a thin YSZ electrolyte, made using processes known to those of skillin the art. An example of such a process is tape-calendaring. A thinYSZ/NiO (80%/20% weight ratio) structural pattern is screen-printed onthe outer surface of the support anode at a thickness ranging from about1 microns to about 7 microns and a width ranging from about 25 micronsto about 125 microns. After being air-dried, the tape is heated to burnout the organic material and fired at a temperature of about 1300° C. toabout 1400° C. A LSM/YSZ cathode is screen printed on the dense, thinYSZ electrolyte. The entire cell is fired at a temperature of about1000° C. to about 1300° C. to form an electrode-supported solid oxideelectrochemical device.

EXAMPLE 4

A laminar composite electrode comprises a cathode-supported bilayertape. The cathode-supported bilayer tape comprises a thick cathode ofYSZ-LSM and a thin YSZ electrolyte, made using processes known to thoseof skill in the art. An example of such a process is tape-calendaring. Athin YSZ structural pattern is screen-printed on the outer surface ofthe support cathode at a thickness ranging from about 1 microns to about7 microns and a width ranging from about 25 microns to about 125microns. After being air-dried, the tape is heated to burn out theorganic material and fired at a temperature of about 1200° C. to about1300° C. A NiO/YSZ anode is screen printed on the dense, thin YSZelectrolyte. The entire cell is fired at a temperature of about 1000° C.to about 1300° C. to form an electrode-supported solid oxideelectrochemical device.

EXAMPLE 5

A YSZ-NiO anode is formed using a known process, such astape-calendaring and tape casting, and fired at a moderate temperatureof about 1000° C. to about 1200° C. to form a prefired anode. On a firstside of the prefired anode a thin YSZ electrolyte layer with a thicknessof approximately 10 microns is screen-printed and dried. On a secondside of the prefired anode a thin YSZ structural pattern isscreen-printed at a thickness ranging from about 1 microns to about 10microns and a width ranging from about 25 microns to about 125 microns.After being air-dried, the assembly is heated and fired at a temperatureof about 1200° C. to about 1400° C. A LSM/YSZ cathode is thenscreen-printed on the dense YSZ electrolyte layer surface. The entirecell is fired at a temperature of about 1000° C. to about 1300° C. toform an electrode-supported solid oxide electrochemical device.

It should be understood that the foregoing relates to particularembodiments of the present invention, and that numerous changes may bemade therein without departing from the scope of the invention asdefined from the following claims.

1. The method of making an electrode-supported electrochemical devicecomprising the steps of: fabricating an electrode-supported bilayercomprising a porous support electrode and a thin and dense electrolyteadjacent to and contiguous with a first side of the porous supportelectrode; fabricating a thin and patterned structure layer having athickness in the range of about 1 micron to about 40 microns onto asecond side of the porous support electrode of the electrode-supportedbilayer to produce a laminar composite electrode, wherein thefabricating comprises coating, spraying, or printing the thing andpatterned structure layer; co-firing the laminar composite electrode;and fabricating a second electrode on top of the thin and denseelectrolyte and opposite the porous support electrode.
 2. The method ofclaim 1 wherein the thin and patterned structure layer has a patternstructured for providing a predetermined mass transport and electricalflow path through the porous support-electrode.
 3. The method of claim 2wherein the thin patterned structure layer comprises an array of landareas and open areas.
 4. The method of claim 1 wherein the bilayer isfabricated in green stage by a technique selected from the groupconsisting of dry processing, tape casting, tape calendaring, andscreen-printing.
 5. The method of claim 1 wherein the step offabricating the thin and patterned structure layer comprises a techniqueselected from the group consisting of screen-printing, spraying, andslurry coating.
 6. The method of claim 5 wherein the step of fabricatingthe laminar composite electrode further comprises the steps of:air-drying the laminar composite electrode; and heating the laminarcomposite electrode.
 7. The method of claim 1 further comprisingelectrically connecting a first metallic interconnect to the laminarcomposite electrode and a second metallic interconnect to the secondelectrode.
 8. The method of claim 1 wherein the second electrodecomprises a cathode and the porous support electrode comprises an anode.9. The method of claim 1 wherein the porous support electrode has athickness in the range of about 100 microns to about 3000 microns. 10.The method of claim 1 wherein the dense, thin electrolyte has athickness in the range of about 5 microns to about 40 microns.
 11. Themethod of claim 1 wherein the porous support electrode comprises amaterial selected from the group consisting of lanthanum strontiummanganites, praseodymium strontium manganites, lanthanum strontiumferrites, lanthanum strontium cobaltites, manganese-(Co, Cr, Ni)spinels, and a conductive pervoskite in the general form of ABO₃, whereA comprises at least one of the elements selected from the groupconsisting of La, Ce, Pr, Sr, Ca, and Ba; and B comprises at least oneof the elements selected from the group consisting of Mn, Fe, Co, Ni,Cu, and Zn.
 12. The method of claim 1 wherein the porous supportelectrode comprises a material selected from the group consisting ofnickel cermet, copper/ceria cermet, and conducting ceramics.
 13. Themethod of claim 1 wherein the electrolyte comprises a material selectedfrom the group consisting of doped zirconia, doped ceria, dopedlanthanum gallate, and doped Ba(Sr)Ce(Zr)O₃.
 14. The method of claim 1wherein the thin patterned structure layer comprises materials selectedfrom the group consisting of doped zirconia, doped ceria, dopedlanthanum gallate, doped Ba(Sr)Ce(Zr)O₃, TiO₂, Al₂O₃, MnO_(x), MgO, andNiO.
 15. The method of claim 7 wherein the first and second metallicinterconnects are made of electrically conducting material and areelectrically contacted with the first and second electrodes byconductive metal/ceramic materials in the form of a mesh, foam, felt, orpaste.
 16. The method of claim 1 wherein the second electrode comprisesan anode and the porous support electrode comprises a cathode.